Multicenter evaluation of a new commercial assay for detection of immunoglobulin M antibodies to Toxoplasma gondii. Multicenter Study Group

Performance characteristics of six IMMULITE 2000 TORCH assays

TORCH is an acronym for Toxoplasma gondii (Toxo), other microorganisms (eg, syphilis), rubella virus (RV), cytomegalovirus (CMV), and herpes simplex virus (HSV) that are associated with congenital abnormalities from maternal infection. We evaluated linearity, imprecision, and comparison with commercially available methods of the IMMULITE 2000 (Diagnostic Products, Los Angeles, CA) Toxo IgG, Toxo IgM, RV IgG, RV IgM, CMV IgG, and HSV IgG assays. Linearity and imprecision results were acceptable. The IMMULITE 2000 assays show good concordance with other commercially available methods except for Toxo IgM and RV IgM. Toxo IgM showed better concordance with a consensus of 3 of 4 (Access, Beckman Coulter, Fullerton, CA; IMMULITE 2000; Platelia, Bio-Rad Laboratories Diagnostics Group, Redmond, WA; and Vidas, bioMerieux, Hazelwood, MO) assays than with Access alone. The RV IgM assay showed better concordance with the Zeus method than with the Diamedix method (Diamedix, Miami, FL). The IMMULITE 2000 TORCH assays studied show acceptable performance and are suitable for routine clinical use. Some commercial assays for Toxo IgM and RV IgM show rather poor concordance.

Prospective study of IgM to Toxoplasma gondii on Beckman Coulter's Access(TM) immunoassay system and comparison with Zeus ELISA and gull IFA assays

We compared a new assay for Toxoplasma IgM on the Access analyzer (Beckman Coulter, Inc., Chaska, MN, USA), a random access instrument based on the principle of paramagnetic particle enzyme immunoassay with an enzyme-linked immunosorbent assay (ELISA) (Zeus Scientific, Inc., Raritan, NJ, USA) and an immunofluorescent assay (IFA) (Gull Laboratories, Inc., Salt Lake City, UT, USA). Four hundred fresh, unfrozen clinical samples from pregnant women (n = 154), HIV positive patients (n = 41), and patients in whom infection with Toxoplasma gondii was suspected (n = 200) were collected and assayed over a three month period. The specificity of the Access assay was compared to the consensus results. Results that were discrepant between the ELISA and IFA were resolved using a third IFA (Zeus). Once resolved, the specificity for the Access assay, the Zeus ELISA and the Gull IFA were 99.22%, 97.91%, and 99.45%, respectively. We conclude that the Access assay specificity is comparable to consensus results, minimizing false positive results; and because it is a random access instrument, it may be preferable over batch methods.

Evaluation of the second generation IMx Toxo IgG antibody assay for detection of antibodies to Toxoplasma gondii in human sera

For an evaluation of the Abbott IMx Toxo IgG second generation, antibodies to Toxoplasma gondii were detected by Abbott IMx Toxo IgG and IgM, Vidas Toxo IgG and Toxo IgM (bioMérieux, France) with immunofluorescence assay verified by the dye-test for IgG, and immunosorbent agglutination assay (ISAGA) for IgM as references. The study included 507 serum samples collected over one month in two laboratories, 32 samples from HIV-infected patients, and 70 serial samples from 23 women surveyed for seroconversion or persistent IgM. After exclusion of nine equivocal results from the 507 samples, the sensitivity and specificity, respectively, were 100% (156/156) and 100% (342/342) for the IMx Toxo IgG and 98.1% (153/156) and 100% (342/342) for the Vidas Toxo IgG. Of the 32 HIV-infected patient samples, 7 gave false positive results with IMx Toxo IgG. This was because the samples had been heated. In 5 of the 70 serial samples. IMx Toxo IgG gave positive results earlier than Vidas Toxo IgG and in two cases earlier than IgM antibody assays. In this study IMx Toxo IgG second generation showed an increase in sensitivity and specificity in comparison with data reported previously for the first generation.

Purification of a recombinant protein expressed in yeast: optimization of analytical and preparative chromatography

The industrial production of recombinant proteins requires control of both fermentation and purification steps. For the serodiagnosis of toxoplasmosis, the main antigen is a membrane protein of 30 kDa (P30). The P30 gene was cloned and expressed in Schizosaccharomyces pombe at 0.7 microg/ml in culture medium. Batch fermentation was optimized by the specific choice of peptones, which enabled optimum growth and protein expression without reducing the efficacy of the purification step. Analytical purification was then carried out using cation-exchange chromatography. For larger volumes, scaling up was performed on expanded mode by using a Streamline system (Pharmacia). This purification step allowed us to obtain a 67.5% recovery with a purification factor greater than 27-fold. Expanded bed adsorption technology is a convenient and effective technique for protein capture directly from feedstock, and the eluted fraction is ready for a second affinity chromatography step. This second step is performed with a yield of 40% and provides a final purification factor of 2000-fold.

The probability of acquiring primary Toxoplasma infection in HIV-infected patients: results of an 8-year retrospective study

There is much hope that HIV-infected patients and AIDS patients can reckon with a prolonged survival in future. The increased survival of AIDS patients with positive Toxoplasma serology is not necessarily associated with an increased risk of developing Toxoplasma encephalitis. For HIV-infected patients with negative Toxoplasma serology, the probability of acquiring a primary Toxoplasma infection in highly endemic areas such as Germany had not been studied to date. One hundred eighty-three HIV-infected patients were followed up between 1987 and 1995 in a retrospective study. Within the cohort, 95% of the patients were male and 83% haemophiliacs. The initial (1987) and final (1995) prevalence rate of Toxoplasma antibodies was 33.3% and 36.6%, respectively. The annual rise of the primary infection rate was calculated as 0.41%. The dye test was used for the detection of Toxoplasma-specific antibodies. This assay proved to be reliable and stable during long-term observation. The rate of primary toxoplasmosis found in this long-term study was not higher than that of pregnant women in Germany. Chemoprophylactic measurements for seronegative HIV-infected patients are therefore not recommended, but regular serological screening to detect seroconverters is.

Evaluation of six commercial kits for detection of human immunoglobulin M antibodies to Toxoplasma gondii. The FDA Toxoplasmosis Ad Hoc Working Group

As a result of reports received by the Food and Drug Administration (FDA) of false-positive results obtained with FDA-cleared in vitro diagnostic kits for the detection of Toxoplasma-specific human immunoglobulin M (IgM) antibodies, an FDA-sponsored evaluation of six kits was performed. A battery of 258 serum specimens, including 30 specimens drawn 1 to 5 months after initial Toxoplasma infection and 228 specimens from Toxoplasma IgG-positive individuals, Toxoplasma IgG-negative individuals, rheumatoid factor-positive persons, and persons determined to be Toxoplasma IgM positive by commercially available assays, was assembled, randomly assorted, and coded. The battery was tested at the FDA with six commercially available kits, at the Palo Alto Medical Foundation (PAMF) by the PAMF double-sandwich IgM enzyme-linked immunosorbent assay (PAMF IgM ELISA), and at the Centers for Disease Control and Prevention (CDC) by the CDC EIA IgM. The results of the PAMF IgM ELISA that were obtained with the battery were considered to be the "gold standard" for this study; specificity rates were computed by considering the PAMF results to be 100% specific. Sensitivity and specificity rates were found to be as follows: CDC EIA IgM, 100 and 99.1%, respectively; Abbott IMx Toxo IgM, version 1, 100 and 77.5%, respectively; Abbott IMx Toxo IgM, version 2, 93.3 and 97.3%, respectively; Abbott Toxo-M EIA, 100 and 84.2%, respectively; BioMérieux Vitek VIDAS Toxo IgM, 100 and 98.6%, respectively; BioWhittaker Toxocap-M, 100 and 95.9%, respectively; Gull Toxo IgM, 97 and 85.6%, respectively; and Sanofi Diagnostics Pasteur Platelia Toxo IgM, 100 and 96.8%, respectively. Although the extent of false-positive reactions with these kits cannot be calculated because the study was retrospective and sample choices were biased, the results may be useful as an indicator of the relative specificities of these kits.

False-positive results in immunoglobulin M (IgM) toxoplasma antibody tests and importance of confirmatory testing: the Platelia Toxo IgM test

Although tests for detection of immunoglobulin M (IgM) toxoplasma antibodies have been reported to have a high degree of accuracy, it is well recognized by investigators in the United States and Europe that false-positive results may occur with many of these tests, at times to an alarming degree. Unfortunately, this information is not well documented in the literature. Studies on various toxoplasma IgM test kits are frequently flawed. The investigators often use reference tests which have not previously been carefully evaluated as well as sera that were not appropriate to answer the question of how often false-positive results might occur. We recently had the unique opportunity to evaluate the accuracy of the Platelia Toxo IgM test in 575 serum samples obtained during an outbreak of toxoplasmosis which occurred in 1995 in the Capital Regional District of British Columbia, Canada. When compared with results obtained in a reference IgM enzyme-linked immunosorbent assay (ELISA), the Platelia Toxo IgM test had a sensitivity of 99.4%, specificity of 49.2%, positive predictive value of 51.9%, negative predictive value of 99.3%, and an overall agreement of 67.0%. In an attempt to resolve discrepancies between these two tests, a serological profile (Sabin-Feldman dye test, IgA and IgE antibody tests, differential agglutination [AC/HS] test, and IgG avidity method) was performed. Of 153 serum samples that were positive in the Platelia Toxo IgM test and negative in the IgM ELISA, 71 (46.4%) were negative in the Sabin-Feldman dye test. Of the serum samples that were positive in the dye test, 77 (93.9%) had a serological profile most compatible with an infection acquired in the distant past. These results reveal high numbers of false-positive results in the Platelia Toxo IgM test and highlight the importance of appropriate evaluation of commercial tests that are currently being marked. Our results also emphasize the importance of confirmatory testing to determine whether the results of an IgM antibody test reflect the likelihood of a recently acquired infection.

Study of Abbott Toxo IMx system for detection of immunoglobulin G and immunoglobulin M toxoplasma antibodies: value of confirmatory testing for diagnosis of acute toxoplasmosis

We compared the Abbott Toxo immunoglobulin G (IgG) and IgM IMx assays with the Sabin-Feldman dye test and an IgM enzyme-linked immunosorbent assay (ELISA) in 398 serum samples previously tested in our laboratory (retrospective group) and 1,000 consecutive serum samples, tested as they were received in our laboratory in 1995 (prospective group). In the retrospective group, the IgG IMx had a sensitivity of 100%, specificity of 99.0%, positive predictive value of 99.0%, negative predictive value of 100%, and overall agreement of 99.5%. The percentages for the IgM IMx were 97.8, 99.0, 98.9, 98, and 98.4%, respectively. In the prospective group, the IgG IMx had a sensitivity of 97.8%, specificity of 98.7%, positive predictive value of 97.8%, negative predictive value of 98.7%, and overall agreement of 98.4%. The percentages for the IgM IMx were 88.3, 95.9, 74.7, 98.3, and 95%. A serological profile (IgA and IgE antibodies and the differential agglutination [AC/HS] test) was performed in an attempt to resolve discrepancies. Of 21 serum samples that were positive in the IgM IMx and negative in the IgM ELISA, 19 had serological profiles which were most compatible with an infection acquired in the distant past. Of 8 serum samples which were positive in the IgM ELISA and negative in the IgM IMx, 5 had serological profiles which were most compatible with an infection acquired in the distant past. Of the 55 serum samples that were positive in both IgM tests, 21 were from patients who had serological profiles which were most compatible with an infection acquired in the distal past. In conclusion, our data highlight the importance of confirmatory testing for the diagnosis of recently acquired infection with Toxoplasma gondii. When compared with the dye test and IgM ELISA, the Toxo IgG and IgM IMx assays, respectively, revealed high overall agreement in the retrospective and prospective study.